There are cards that do that - e.g. some of the merchants have store cards that give you immediate discount - e.g. I have one that makes 5% off anything I buy in a particular big store chain. I guess for airlines it is worth it even more since airline tickets cost hundreds of dollars, so if they can avoid paying couple of percents of each transaction to intermediaries by having their own card brand that adds up to serious money, and since they control points program they can give out points so that it's still profitable to them.